C++ what is the consteval? How is it different to const and constexpr?

C++: What is consteval? How is it Different from const and constexpr?

If you’ve been using const and constexpr in your C++ code, you might be wondering what the new consteval keyword in C++20 brings to the table. Let’s break it down clearly.

What is consteval?

The consteval specifier declares a function as an immediate function, meaning the function must be evaluated at compile time. Unlike constexpr, which allows both compile-time and runtime evaluation, consteval enforces compile-time only.

consteval int square(int x) {
    return x * x;
}

Any attempt to call this function with non-constant input (or at runtime) will cause a compile-time error.

Example

consteval int add(int a, int b) {
    return a + b;
}

int main() {
    constexpr int result = add(2, 3);  // OK
    int x = 5;
    // int y = add(x, 3);  // ERROR: add must be evaluated at compile time
}

Comparison Table

When Should You Use consteval?

• When you want 100% guarantee that a function runs only at compile time.
• For validating inputs during compile-time metaprogramming.
• To prevent any accidental runtime overhead.

Advanced Use: Compile-Time String Length

consteval std::size_t const_strlen(const char* str) {
    std::size_t len = 0;
    while (str[len] != '\0') ++len;
    return len;
}

constexpr auto len = const_strlen("Hello");  // OK

Summary

consteval is a powerful addition to C++ for strict compile-time enforcement. Use it when constexpr isn’t strict enough, and you want full control over compile-time behavior.

C/C++ Programming

• Understanding std::transform_reduce in Modern C++
• Implement a Lock Acquire and Release in C++
• Detecting Compile-time vs Runtime in C++: if consteval vs std::is_constant_evaluated()
• C++ Forward References: The Key to Perfect Forwarding
• Understanding dynamic_cast in C++: Safe Downcasting Explained
• C vs C++: Understanding the restrict Keyword and its Role in Optimization
• C++ Lvalue, Rvalue and Rvalue References
• C++ assert vs static_assert
• Why auto_ptr is Deprecated in C++?
• C++ What is the consteval? How is it different to const and constexpr?
• Tutorial on C++ std::move (Transfer Ownership)
• const vs constexpr in C++
• Tutorial on C++ Ranges
• Tutorial on C++ Smart Pointers
• Tutorial on C++ Future, Async and Promise
• The Memory Manager in C/C++: Heap vs Stack
• The String Memory Comparision Function memcmp() in C/C++
• Modern C++ Language Features
• Comparisions of push_back() and emplace_back() in C++ std::vector
• C++ Coding Reference: is_sorted_until() and is_sorted()
• C++ Coding Reference: iota() Setting Incrementing Values to Arrays or Vectors
• C++ Coding Reference: next_permutation() and prev_permutation()
• C++ Coding Reference: count() and count_if()
• C++ Code Reference: std::accumulate() and Examples
• C++ Coding Reference: sort() and stable_sort()
• The Next Permutation Algorithm in C++ std::next_permutation()

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